Segmentation fault from free()

[Lei Yang <leiyang@nec-labs.com>]

Hi friends,

I am writing a c code and have been bugged by this segmentation fault 
for a while.

What I did is simply like:

-----------------------------------------------------------------------------------------------------
.........
unsigned long blocksize = 2048;
char *in_buffer
char *out_buffer;
if(( in_buffer= malloc(blocksize)) == NULL)
{
fprintf(stderr, "*** Can't malloc(%ld) forbuffer.\n",blocksize);
return NULL;
}

if(( out_buffer= malloc(2*blocksize)) == NULL)
{
fprintf(stderr, "*** Can't malloc(%ld) forbuffer.\n",blocksize);
free(in_buffer);
return NULL;
}

loop: until all the data are read from file
{
//read a block of data from a file to in_buffer
// do some data processing with in_buffer
//write the result to out_buffer
//memcpy out_buffer to list
}

free(in_buffer);
free(out_buffer);

return list;
......
-----------------------------------------------------------------------------------------------------------------

I've debugged with gdb to see where the segmentation fault happens, it 
is at free(in_buffer).
I've verified that the value for in_buffer after malloc() and before 
free() is the same. Or in other words, in_buffer is a valid pointer 
allocated by malloc.

And the SF only happens when the file is large, although block size 
could be small.
Means that for both small (2KB) and large(5MB) files, block size are the 
same. However, only large files could cause SF.

Could anyone please point me out what could possibly be the reason?
BTW, pls cc me when you reply, since I am not able to receive emails 
from this list. Thanks a lot!

TIA!
Lei